In mobile communication systems, capacity and performance are usually limited by multipath and co-channel interference. Multipath is a condition which arises when a transmitted signal undergoes reflection from various obstacles in the propagation environment. The multipath signals follow different paths and have different phases when they are arrive at the receiver. The result is degradation in signal quality when they are combined at the receiver due to the phase mismatch.
Smart antennas enable a higher capacity in wireless networks by effectively reducing multipath and co-channel interference. Smart antennas focus the radiation in the preferred direction and adjusts itself to changing traffic conditions or signal environments. The signals from these elements are combined to form a movable or switchable beam pattern. The process of combining the signals and then focusing the radiation in a particular direction is often referred to as digital beamforming.
By way of example and not of limitation, there are two types of smart antennas that dynamically change their antenna pattern to mitigate interference and multipath effects while increasing coverage and range, namely, switched beam and adaptive arrays. The switched beam smart antenna system provides an increase in network capacity with an antenna array that generates beams that cover specific areas. For an illustrative base station, the base station determines the beam that is best aligned in the signal-of-interest direction and then switches to that beam to communicate with the mobile station.
By way of example and not of limitation, the adaptive array smart antenna system tracks the mobile user continuously by steering the main beam towards the mobile station and at the same time forming nulls in the directions of the interfering signal. In the illustrative example, the signal received from each of the spatially distributed antenna elements is multiplied by a weight. The weights are complex in nature and adjust the amplitude and phase. These signals are combined to yield the array output. These complex weights are computed by an adaptive algorithm.
There are a variety of benefits to the use of smart antennas which include the reduction of co-channel interference, range improvement, increase in capacity, reduction in transmitted power, mitigation of multi-path effects and compatibility with TDMA, FDMA and CDMA systems.
However, there are a number of limitations to smart antennas. These include performance degradation when the mobile station is in motion. More particularly, at driving speeds and pedestrian speeds, performance degradation is caused by inadequate beam steering. Beam steering is the changing of the direction of the main lobe of a radiation pattern. In radio systems, beam steering may be accomplished by switching antenna elements or by changing the relative phases of the RF signals.
Performance degradation is caused by the smart antenna's limitations. The smart antenna's beam steering is too focused, and does not easily accommodate movement of the mobile station. As a result the mobile station may drop calls. Solutions such as increasing processor speed fail to solve these smart antenna limitations because the problem revolves around measurements needed to collect the data required for beam steering. The measurements that might be performed for beam steering include power levels, signal to noise ratios, power control and other such measurements that are performed during the beam steering process. Each of these measurements must be made over a period of time to provide sufficient accuracy. However, when the mobile station is in motion, the time needed to make these measurements with the required accuracy is not available. The resulting measurement data is less accurate or is incomplete resulting in performance degradation of the smart antenna. Thus, when the mobile station is moving faster than the rate at which the smart antenna measurements can be completed, performance is degraded.